Lithium battery separator comprising porous pvdf-based resin coating and preparation method therefor

Abstract

A lithium battery separator comprising a porous PVDF-based resin coating. The porous PVDF-based resin coating is located on at least one surface of a base film; and the thickness of a single porous PVDF-based resin coating is 0.5-3.5 μm, wherein the bonding strength (N / M) / coating air permeability increment (s / 100cc) ≥ 0.25, and the bonding strength (N / M) / areal density per unit coating (g / m2 / μm) ≥ 10, so that a porous PVDF-based resin coating having excellent thickness, coating air permeability increment, bonding strength, and thermal shrinkage is obtained. The formed lithium battery separator also has excellent cycling performance and heat resistance.

Description

Lithium battery separator comprising porous PVDF resin coating and preparation method thereof

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This disclosure claims priority to the Chinese patent application with application number CN 2021116358562 filed with the Chinese Patent Office on December 30, 2021, entitled “Lithium battery separator comprising a porous PVDF-based resin coating and a method for preparing the same,” the entire contents of which are incorporated by reference into this disclosure. Technical Field

[0003] The present disclosure relates to a lithium battery separator and a lithium battery, and particularly to a lithium battery separator comprising a porous PVDF-based resin coating and a lithium battery thereof. Background Art

[0004] In lithium batteries, the main function of the diaphragm is to separate the positive and negative electrodes of the battery to prevent the two electrodes from contacting and short-circuiting. In addition, it also has the function of allowing electrolyte ions to pass through. From the perspective of thermal stability, easy formation of a porous adhesive coating, and electrolyte wettability, the existing technology usually uses polyvinylidene fluoride (PVDF) resin as a bonding layer. In order to improve the unit energy density of lithium batteries, it is usually hoped that the thickness of the diaphragm is as thin as possible. In order to improve the adhesion between the diaphragm and the positive and negative active materials, there are also higher requirements for the type and thickness of the bonding layer. In addition, the diaphragm has requirements for ion conductivity, thermal shrinkage and mechanical strength. How to further design the polyvinylidene fluoride (PVDF) resin bonding layer is particularly important.

[0005] Summary of the Invention

[0006] Therefore, to date, the existing technology has not focused on how to balance the performance of the four aspects of coating thickness, bonding strength, thermal shrinkage and ion conductivity for lithium battery separators containing porous PVDF resin coatings, thereby improving the performance of lithium batteries.

[0007] To address this technical problem, the inventors conducted systematic research and ultimately discovered that by limiting the ratio of bonding strength to coating permeability value-added, and the ratio of bonding strength to surface density per unit thickness, within specific ranges, a porous PVDF resin coating with excellent thickness, coating permeability value-added, bonding strength, and thermal shrinkage can be obtained. The resulting lithium battery separator also exhibits excellent cycling performance and heat resistance. The specific solutions disclosed in this disclosure are as follows:

[0008] 1. A lithium battery separator comprising a porous PVDF resin coating, wherein the porous PVDF resin coating is located on at least one surface of a base film, the thickness of the porous PVDF resin coating on one side is 0.5-3.5 μm, the bonding strength (N / m) / coating air permeability value (s / 100cc) is ≥ 0.25, and the bonding strength (N / m) / unit coating surface density (g / m 2 / μm)≥10.

[0009] 2. The lithium battery separator according to technical solution 1, wherein the PVDF resin in the porous PVDF resin coating includes high molecular weight PVDF and low molecular weight PVDF, wherein the weight average molecular weight of the high molecular weight PVDF is 800,000-1.2 million, the weight average molecular weight of the low molecular weight PVDF is 200,000-500,000, and the weight ratio of the high molecular weight PVDF to the low molecular weight PVDF is 7:1-1:7.

[0010] 3. The lithium battery separator according to one of technical solutions 1 and 2, wherein the porous PVDF-based resin coating may further include inorganic particles, and the content of the inorganic particles is 40-80 wt% based on the total weight of the porous PVDF-based resin coating.

[0011] 4. The lithium battery separator according to any one of technical solutions 1-3, wherein the water content of the slurry of the porous PVDF resin coating is less than 5 wt% during the preparation process.

[0012] 5. The lithium battery separator according to any one of technical solutions 1-4, wherein during the preparation of the porous PVDF-based resin coating, the water content of the coagulation bath is 30 wt%-70 wt%.

[0013] 6. The lithium battery separator according to one of technical solutions 1-5, wherein the porous PVDF resin coating is dried at a temperature of 60-100°C during the preparation process.

[0014] 7. A method for preparing a lithium battery separator comprising a porous PVDF resin coating, comprising the following steps:

[0015] Step 1: dispersing a PVDF resin and optional inorganic particles in a solvent to form a coatable slurry, wherein the solvent includes a good solvent and the water content of the entire slurry is less than 5 wt %;

[0016] Step 2: coating the coatable slurry on at least one surface of the base film, then immersing the base film in a room temperature coagulation bath to coagulate the coating to form a porous PVDF resin coating, and then washing and drying the coating to obtain a lithium battery separator;

[0017] The water content in the coagulation bath is 30wt%-70wt%; and the drying temperature is 60-100°C.

[0018] 8. The preparation method according to technical solution 7, wherein the PVDF-based resin includes high molecular weight PVDF and low molecular weight PVDF, wherein the weight average molecular weight of the high molecular weight PVDF is 800,000-1.2 million, the weight average molecular weight of the low molecular weight PVDF is 200,000-500,000, and the weight ratio of the high molecular weight PVDF to the low molecular weight PVDF is 7:1-1:7.

[0019] 9. The preparation method according to technical solution 7, wherein the content of the inorganic particles is 40-80 wt% based on the total weight of the porous PVDF-based resin coating.

[0020] 10. A lithium battery comprising a positive electrode, a negative electrode, a non-aqueous electrolyte and a lithium battery separator according to one of technical solutions 1-6 or a lithium battery separator obtained according to the method according to one of technical solutions 7-9.

[0021] Beneficial effects of the present disclosure:

[0022] The lithium battery separator obtained according to the present disclosure, which comprises a porous PVDF resin coating, has excellent positive electrode bonding performance, high-temperature dimensional stability, and good air permeability, so that the final lithium battery has high energy density and excellent cycle performance, thereby solving the technical problem in the prior art that the bonding strength and ion conductivity of the separator cannot be taken into account at the same time. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] FIG1 is a SEM image showing the surface morphology of the lithium battery separator including a porous PVDF-based resin coating in Example 2.

[0024] FIG. 2 is a SEM image showing the apparent cross section of the lithium battery separator including the porous PVDF-based resin coating in Example 2.

[0025] FIG3 is a SEM image showing the surface morphology of the lithium battery separator including the porous PVDF-based resin coating in Example 12. DETAILED DESCRIPTION

[0026] In order to better explain the present disclosure, the present disclosure will be described in detail with reference to the embodiments of the present disclosure, and the main contents of the present disclosure will be further illustrated in conjunction with specific examples. However, the content of the present disclosure is not limited to the following examples. Where specific techniques or conditions are not specified in the examples, the techniques or conditions described in the literature in the field or in accordance with the product instructions are used. Where the manufacturer of the reagents or instruments is not specified, they are all conventional products that can be obtained commercially.

[0027] [Basement membrane]

[0028] The base membrane is usually a polyolefin porous base membrane, which can be selected from the polyolefin porous base membranes previously suitable for lithium battery separators, including one or more copolymers or multiple blends selected from polyethylene, polypropylene, polybutene, and poly-4-methylpentene.

[0029] From the viewpoint of exhibiting the shutdown function, the polyolefin microporous membrane preferably contains polyethylene, and the content of polyethylene is preferably 95% by mass or more of the total content of the base film.

[0030] In one embodiment, the polyolefin porous base membrane is a single-layer polyolefin microporous membrane. In another embodiment, the polyolefin porous base membrane is a polyolefin microporous membrane having a stacked structure of two or more layers.

[0031] The polyolefin contained in the polyolefin porous base membrane preferably has a weight-average molecular weight (Mw) of 100,000 to 5,000,000. A weight-average molecular weight of 100,000 or greater ensures adequate mechanical properties. On the other hand, a weight-average molecular weight of 5,000,000 or less provides excellent shutdown properties and facilitates film formation.

[0032] The thickness of the polyolefin porous base membrane is not particularly limited, but is preferably 5 to 30 μm. The polyolefin porous base membrane is a porous polymer membrane formed mainly by stretching.

[0033] There is no limitation on the method for manufacturing the polyolefin porous base film according to the exemplary embodiment of the present disclosure, as long as the polyolefin porous base film is manufactured by a person skilled in the art. In an exemplary embodiment, the polyolefin porous base film can be manufactured by a dry process or a wet process. The dry process is a method for forming micropores by forming a polyolefin film and then stretching the film at low temperature, wherein the stretching causes microcracks between the thin sheets that are the crystalline parts of the polyolefin. The wet process is a method in which a polyolefin-based resin and a diluent are mixed at a high temperature at which the polyolefin-based resin is melted to form a single phase, the polyolefin and the diluent are phase separated during the cooling process, and then the diluent is extracted to form pores therein. The wet process is a method in which mechanical strength and transparency are imparted by a stretching / extraction process after phase separation treatment. The wet process is more preferred because the film thickness is thin, the pore size is uniform, and the physical properties are excellent compared to the dry process.

[0034] From the viewpoint of obtaining appropriate membrane resistance and shutdown function, the porosity of the porous substrate is preferably 20% to 60%, and the average pore diameter is preferably 15 to 100 nm.

[0035] From the viewpoint of improving the production yield, the puncture strength of the polyolefin porous base film is preferably 200 g or more.

[0036] [Porous PVDF resin coating]

[0037] In the present disclosure, porous PVDF-based resin coating is used as an adhesive layer, wherein the PVDF-based resin is based on all adhesive resins, with a content of more than 90wt%, or more than 95wt%, or more than 99wt%. The PVDF-based resin specifically includes a copolymer of vinylidene fluoride homopolymer, vinylidene fluoride and other copolymerizable monomers, or a mixture thereof. The specific example of the monomer that can be copolymerized with vinylidene fluoride can include at least one selected from the following: tetrafluoroethylene, hexafluoropropylene, trifluoroethylene, chlorofluoroethylene, 1,2-difluoroethylene, perfluoro(methyl vinyl) ether, perfluoro(ethyl vinyl) ether, perfluoro(propyl vinyl) ether, difluoro(1,3-dioxole), perfluoro(2,2-dimethyl-1,3-dioxole), trichloroethylene and vinyl fluoride.

[0038] Controlling the molecular weight of PVDF resins affects the adhesion, porosity, and dimensional stability of PVDF resins. Therefore, the PVDF resins include a high-molecular-weight first PVDF resin and a low-molecular-weight second PVDF resin, wherein the high-molecular-weight PVDF has a weight-average molecular weight of 800,000-1.2 million, the low-molecular-weight PVDF has a weight-average molecular weight of 200,000-500,000, and the weight ratio of high-molecular-weight PVDF to low-molecular-weight PVDF is 7:1-1:7. Based on the final effect, the high-molecular-weight first PVDF resin may have a greater contribution to the adhesion and de-embedding of inorganic particles and dimensional stability, while the low-molecular-weight second PVDF resin may have an impact on film-forming properties, porosity, and interfacial adhesion.

[0039] The porous PVDF resin coating preferably further contains inorganic particles, and any inorganic filler that is stable and electrochemically stable relative to the electrolyte can be selected. Specifically, one or more of the following materials can be selected:

[0040] Alumina, silica, titania, zirconia, magnesia, ceria, titanium dioxide, zinc oxide, and iron oxide in oxide-based ceramics;

[0041] Nitride-based materials: silicon nitride, titanium nitride, and boron nitride;

[0042] Metal hydroxide-based aluminum hydroxide, magnesium hydroxide, calcium hydroxide, chromium hydroxide, zirconium hydroxide, cerium hydroxide, nickel hydroxide;

[0043] and boehmite, magnesium carbonate, magnesium sulfate, barium sulfate, calcium carbonate, wollastonite, silicon carbide, etc., among which alumina or boehmite is preferred.

[0044] Based on the total weight of the porous PVDF-based resin coating, the content of the inorganic particles is 40-80 wt %, more preferably 50-75 wt %, and even more preferably 60-70 wt %.

[0045] [Slurry]

[0046] To obtain a porous PVDF resin coating, a coatable slurry must first be prepared by combining the PVDF resin, optional inorganic particles, and a good solvent. Examples of good solvents include acetone, methyl ethyl ketone, N-methylpyrrolidone, and polar amide solvents such as dimethylacetamide, diethylformamide, and diethylformamide. Water is not required in the slurry as a poor solvent for phase separation. In fact, the water content in the overall slurry is preferably below 5 wt%.

[0047] From the viewpoint of forming a good porous structure, the content of the PVDF-based resin and the inorganic particles in the slurry is preferably 6 wt % to 20 wt %, preferably 8 wt % to 15 wt %, with the remainder being a good solvent.

[0048] [Coating method]

[0049] The coatable slurry is applied to at least one surface of a base film, preferably by first applying the coatable slurry to the porous substrate to form a coating film by dip coating, air knife coating, curtain coating, roll coating, wire bar coating, gravure coating, or die coating. Of these coating methods, gravure coating or die coating is preferred as the coating method for the coating liquid.

[0050] The base film coated with the slurry is treated with a coagulation liquid capable of coagulating the above-mentioned PVDF resin. Thus, the PVDF porous resin is coagulated to form a porous PVDF resin layer. As a method for treating with a coagulation liquid, there can be cited a method of spraying the coagulation liquid on the substrate coated with the coating slurry, a method of immersing the substrate in a bath (coagulation bath) to which a coagulation liquid is added, and the like. There are no particular restrictions on the coagulation liquid, as long as it is a liquid capable of coagulating the PVDF resin, preferably water or a solution obtained by mixing an appropriate amount of water with the solvent used in the slurry. The water content in the coagulation bath is 30wt%-70wt%, preferably 35wt%-65wt%, and more preferably 40wt%-60wt%. The coagulation bath temperature is room temperature.

[0051] Then, the lithium battery separator containing the porous PVDF resin coating that has passed through the coagulation bath is dried. As for the drying conditions, as long as the base film does not shrink due to softening and the PVDF resin and the inorganic particles can be fully bonded, there is no particular restriction on the wind speed and drying temperature. As drying methods, heat transfer drying (adhesion to high-temperature objects), convection heat transfer (hot air), radiation heat transfer (infrared), and other methods (microwave, induction heating, etc.) can be listed. Among them, in the above-mentioned manufacturing method, since it is necessary to have a precise and uniform drying speed in the width direction, it is preferred to use convection heat transfer or radiation heat transfer. In addition, during constant rate drying, in order to achieve a uniform drying speed in the width direction, when using convection heat transfer drying, it is preferred to use a method that can reduce the total material mobility coefficient during drying while maintaining a controllable wind speed. Specifically, a method of conveying hot air in a direction parallel to the supporting substrate, parallel to the conveying direction of the substrate, or perpendicular to it can be used. The drying temperature is preferably controlled at 60-100°C, preferably 70-90°C, and more preferably 75-85°C.

[0052] Through different embodiments, it is found that different coagulation bath water content and drying temperature have an impact on the air permeability and bonding strength of the coating.

[0053] [Lithium battery]

[0054] The lithium battery disclosed herein comprises a positive electrode, a negative electrode, an electrolyte, and the separator disclosed herein disposed between the positive electrode and the negative electrode. Specifically, the battery element and the electrolyte are sealed together in an external packaging material, and the battery element is obtained by placing the negative electrode and the positive electrode opposite to each other with the separator between them.

[0055] The positive electrode has a structure in which, for example, an active material layer containing a positive electrode active material and a binder resin is formed on a current collector.

[0056] Examples of the positive electrode active material include commonly used positive electrode active materials in the art, such as lithium-containing transition metal oxides, specifically LiCoO2, LiNiO2, LiMn 1 / 2 Ni 1 / 2 O2、LiCo 1 / 3 Mn 1 / 3 Ni 1 / 3 O2, LiMn2O4, LiFePO4, LiCO 1 / 2 Ni 1 / 2 O2、LiAl 1 / 4 Ni 3 / 4O2, etc. Examples of binder resins include polyvinylidene fluoride resins and styrene-butadiene copolymers. A conductive additive may also be included, such as carbon materials such as acetylene black, Ketjen black, and graphite powder. Examples of current collectors include aluminum foil, titanium foil, and stainless steel foil with a thickness of 5 to 20 μm.

[0057] As an embodiment of the negative electrode, there can be cited a structure in which an active material layer containing a negative electrode active material and a binder resin is formed on a current collector. The active material layer may further contain a conductive additive. As the negative electrode active material, there can be cited materials that can electrochemically absorb lithium, specifically, for example: carbon materials; alloys of silicon, tin, aluminum, etc. with lithium; Wood's alloy; and the like. The binder resin, conductive additive, and current collector are substantially the same as those of the positive electrode. In addition, a metal lithium foil can be used as the negative electrode instead of the above-mentioned negative electrode.

[0058] The electrolyte is a solution obtained by dissolving a lithium salt in a non-aqueous solvent. As an embodiment of the electrolyte, it can be an electrolyte system commonly used in the art. Examples of lithium salts include LiPF6, LiBF4, LiClO4, etc. Examples of non-aqueous solvents include cyclic carbonates such as ethylene carbonate, propylene carbonate, fluoroethylene carbonate, difluoroethylene carbonate, and vinylene carbonate; chain carbonates such as dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, vinyl carbonate, and fluorinated derivatives thereof; cyclic esters such as γ-butyrolactone and γ-valerolactone, etc., which can be used alone or in combination.

[0059] Example

[0060] Hereinafter, the separator and the lithium secondary battery including the separator according to the present disclosure will be described in more detail through examples. However, the embodiments of the present disclosure are not limited to the following examples.

[0061] <Evaluation Method>

[0062] (1)Film thickness

[0063] First use a micrometer to test the thickness of the isolation base film, and then test the thickness after coating. The thickness after removing the isolation base film is the thickness of the porous coating.

[0064] (2) Average particle size

[0065] The particle size was measured using a particle size measuring device (Microtrac UPA150, manufactured by Nikkiso Co., Ltd.) under the following measurement conditions: load index = 0.15 to 0.3, measurement time = 300 seconds, and the value of the 50% particle size in the obtained data was recorded as the particle size.

[0066] (3) Average pore size of basement membrane

[0067] The pore size distribution of the porous membrane was tested using the AAQ-3K-A-1 water pressure tester from PMI Instruments, Inc., USA. Water was used as the test liquid. Under pressure, the water was squeezed into the pores of the membrane. The pressure corresponding to the water squeezed into pores of different diameters followed the Washburn equation, thereby calculating a series of pore structure parameters of the membrane.

[0068] (4) Determination of weight average molecular weight of PVDF resin

[0069] PVDF resin was dissolved in DMF at a concentration of 1.0 mg / ml to obtain a sample solution. 50 ml of this sample solution was used to perform GPC measurement under the following conditions to determine its weight average molecular weight (PMMA equivalent).

[0070] Device: HLC-8220GPC (Tosoh Corporation)

[0071] Column: Shodex KF-606M, KF-601

[0072] Mobile phase: 0.6 ml / min DMF

[0073] Detector: Differential refractive index detector

[0074] (5) Gurley air permeability

[0075] A 100 mm x 100 mm sample of the porous membrane was cut and tested using a Gurley 4110N air permeability tester (USA) using a 100 cc test gas mode. The time it took for the test gas to completely pass through the porous membrane sample was recorded, which is the Gurley value. The Gurley value of the porous membrane is the Gurley value of the porous membrane minus the Gurley value of the membrane without a porous membrane (i.e., a pure porous substrate).

[0076] (6) Peel strength between porous substrate and porous layer

[0077] A 12 mm wide, 15 cm long adhesive tape (Scotch, Model 550R-12) was applied to the porous layer of one side of the separator. The separator was cut so that its width and length matched those of the adhesive tape to prepare a measurement sample. The adhesive tape was applied to the separator so that its longitudinal direction aligned with the MD direction of the separator. It should be noted that the adhesive tape served as a support for peeling off the porous layer.

[0078] The measurement sample was left in an atmosphere of a temperature of 23±1° C. and a relative humidity of 50±5% for 24 hours or more, and the following measurements were performed in the same atmosphere.

[0079] The adhesive tape and the porous layer immediately below it are peeled off by about 10 cm, so that the laminate (1) of the adhesive tape and the porous layer is separated from the laminate (2) of the porous substrate and the other porous layer by about 10 cm. The end of the laminate (1) is fixed to the upper chuck of TENSILON (RTC-1210A made by Orientec), and the end of the laminate (2) is fixed to the lower chuck of TENSILON. The measurement sample is suspended in the direction of gravity so that the tensile angle (the angle of the laminate (1) relative to the measurement sample) becomes 180 °. The laminate (1) is stretched at a tensile speed of 20 mm / min, and the load when the laminate (1) is peeled off from the porous substrate is measured. The load from 10 mm to 40 mm after the start of the measurement is obtained at intervals of 0.4 mm, and its average value is used as the peel strength.

[0080] (7) Bond strength

[0081] The test was carried out with reference to the requirements of GB / T 2792.

[0082] 1) stacking A4 paper and a separator in the order of A4 paper / separator / separator / A4 paper, with the separator coatings facing each other;

[0083] 2) The stacked A4 paper and separator are subjected to thermoplastic treatment at a temperature of 100°C;

[0084] 3) Cut the thermoplasticized membrane into strips with a length of 200 mm and a width of 25 mm. The distance between the fixtures is (100 ± 5) mm, and the test speed is (50 ± 10) mm / min.

[0085] (8) Thermal shrinkage

[0086] The test was carried out with reference to the requirements of GB / T12027-2004.

[0087] 1) Cut a diaphragm of 15*15cm in size, and measure the longitudinal and transverse lengths of the sample using a ruler along the markings on the surface of the diaphragm.

[0088] 2) Use a ruler to measure the longitudinal and transverse lengths of the sample;

[0089] 3) Place the sample flat in a fixture and then place it in an oven at 130°C for 60 minutes;

[0090] 4) After heating, remove the sample and wait until it returns to room temperature. Measure the longitudinal and transverse mark lengths again and calculate the shrinkage rate according to the following formula. Finally, take the average value of several samples as the shrinkage rate.

[0091]

[0092] ΔL - thermal shrinkage in the longitudinal direction of the specimen, expressed in %;

[0093] L0 - the length of the sample in the longitudinal direction before heating, in millimeters (mm);

[0094] L is the length of the sample in the longitudinal direction after heating, in millimeters (mm);

[0095] ΔT - thermal shrinkage in the transverse direction of the specimen, expressed in %;

[0096] T0 - the transverse length of the sample before heating, in millimeters (mm);

[0097] T is the transverse length of the sample after heating, in millimeters (mm).

[0098] (9) Diaphragm puncture strength

[0099] Prepare a sheet sample and fix it under the test fixture. Use a high-speed rail tensile tester and a needle fixture. Use a 1mm diameter needle on the puncture tester at a speed of 50mm / min to puncture. After the data stabilizes, measure the top puncture force F. The puncture strength (unit: gf) is calculated as F / 9.8*1000.

[0100] Example 1:

[0101] Based on 100 parts by weight of the coatable slurry, 3 parts by weight of a PVDF resin with a weight average molecular weight of 900,000 to 1.1 million, 2 parts by weight of a PVDF resin with a weight average molecular weight of 400,000 to 500,000, and 7.5 parts by weight of Al2O3 particles were added to a dimethylacetamide (DMAC) / water dispersion system with a water content of 2 wt% and dissolved at 50°C for about 3 hours to form a binder. At 23°C and 20% relative humidity, the slurry for forming a porous coating was coated on both surfaces of a polyethylene porous substrate (thickness 9 μm, porosity 35%, average pore size 70 nm, puncture strength 450 gf) by a gravure roll, coagulated with a dimethylacetamide / water coagulation solution (DMAC / water = 5:5 in the coagulation bath), rinsed with pure water, and dried at 80°C to produce a separator having a porous coating. The resulting two-sided coating had a total thickness of 5.36 μm and a total surface density of 7.24 g / m2. 2 .

[0102] Example 2:

[0103] Based on 100 parts by weight of the coatable slurry, 3 parts by weight of a PVDF resin having a weight average molecular weight of 900,000 to 1.1 million, 1.5 parts by weight of a PVDF resin having a weight average molecular weight of 400,000 to 500,000, and 7.5 parts by weight of Al2O3 particles were added to dimethylacetamide (DMAC) and dissolved at 50°C for about 3 hours to form an adhesive. Under the conditions of 23°C and 20% relative humidity, the slurry for forming a porous coating was coated on both surfaces of a polyethylene porous substrate (thickness 9 μm, porosity 35%, average pore size 70 nm, puncture strength 450 gf) by a gravure roller, coagulated with a dimethylacetamide / water coagulation solution (DMAC / water in the coagulation bath = 5:5), washed with pure water, and dried at 80°C to produce a separator having a porous coating. The resulting total thickness of the two-sided coating was 3.4 μm, and the total surface density of the two-sided coating was 4.92 g / m 2 The surface morphology of the diaphragm is shown in Figure 1, and the cross-sectional morphology is shown in Figure 2.

[0104] Example 3:

[0105] Based on 100 parts by weight of the coatable slurry, 2 parts by weight of a PVDF resin having a weight average molecular weight of 900,000 to 1.1 million, 3 parts by weight of a PVDF resin having a weight average molecular weight of 400,000 to 500,000, and 7.5 parts by weight of Al2O3 particles were added to dimethylacetamide (DMAC) and dissolved at 50°C for about 3 hours to form an adhesive. Under the conditions of 23°C and 20% relative humidity, the slurry for forming a porous coating was coated on both surfaces of a polyethylene porous substrate (thickness 7 μm, porosity 35%, average pore size 70 nm, puncture strength 370 gf) by a gravure roller, coagulated with a dimethylacetamide / water coagulation solution (DMAC / water in the coagulation bath = 5:5), washed with pure water, and dried at 80°C to produce a separator having a porous coating. The resulting total thickness of the two-sided coating was 4.44 μm, and the total surface density of the two-sided coating was 5.14 g / m 2 .

[0106] Example 4

[0107] Based on 100 parts by weight of the coatable slurry, 2 parts by weight of a PVDF resin having a weight average molecular weight of 900,000 to 1.1 million, 3 parts by weight of a PVDF resin having a weight average molecular weight of 400,000 to 500,000, and 7.5 parts by weight of Al2O3 particles were added to dimethylacetamide (DMAC) and dissolved at 50°C for about 3 hours to form an adhesive. Under the conditions of 23°C and 20% relative humidity, the slurry for forming a porous coating was coated on both surfaces of a polyethylene porous substrate (thickness 7 μm, porosity 35%, average pore size 70 nm, puncture strength 370 gf) by a gravure roller, coagulated with a dimethylacetamide / water coagulation solution (DMAC / water in the coagulation bath = 5:5), washed with pure water, and dried at 80°C to produce a diaphragm having a porous coating. Compared with Example 3, the total thickness of the final double-sided coating was controlled to be 6.6 μm, and the total surface density of the double-sided coating was 7.2 g / m 2 .

[0108] Example 5

[0109] Compared with Example 1, the DMAC / water ratio in the coagulation bath is 6:4. The total thickness of the double-sided coating obtained is 4.44 μm, and the total surface density of the double-sided coating is 5.14 g / m 2 .

[0110] Example 6

[0111] Compared with Example 1, the DMAC / water ratio in the coagulation bath is 4:6. The total thickness of the double-sided coating obtained is 5.48 μm, and the total surface density of the double-sided coating is 5.08 g / m 2 .

[0112] Example 7

[0113] Compared with Example 1, the DMAC / water ratio in the coagulation bath is 3:7. The total thickness of the double-sided coating obtained is 4.92 μm, and the total surface density of the double-sided coating is 5.16 g / m 2 .

[0114] Example 8

[0115] Compared with Example 2, the drying temperature was controlled to be 70°C. The total thickness of the double-sided coating was finally obtained to be 3.4 μm, and the total surface density of the double-sided coating was 5.32 g / m 2 .

[0116] Example 9

[0117] Compared with Example 2, the drying temperature was controlled to be 85°C. The total thickness of the double-sided coating was 3.72 μm, and the total surface density of the double-sided coating was 5.06 g / m 2 .

[0118] Example 10

[0119] Compared with Example 2, the drying temperature was controlled to be 100°C. The total thickness of the double-sided coating finally obtained was 4.16 μm, and the total surface density of the double-sided coating was 5.68 g / m 2 .

[0120] Example 11

[0121] Based on 100 parts by weight of the coatable slurry, 0.6 parts by weight of a PVDF resin having a weight average molecular weight of 900,000-1.1 million and 4.2 parts by weight of a PVDF resin having a weight average molecular weight of 400,000-500,000 were added to dimethylacetamide (DMAC) and dissolved at 50° C. for about 3 hours to form an adhesive. Under the conditions of 23° C. and 20% relative humidity, the slurry for forming a porous coating was coated on both surfaces of a polyethylene porous substrate (thickness 9 μm, porosity 35%, average pore size 70 nm, puncture strength 450 gf) by a gravure roller, solidified by dimethylacetamide / water coagulation solution (DMAC / water in the coagulation bath = 5:5), washed with pure water, and dried at 80° C. to produce a diaphragm having a porous coating. Compared with Example 3, the total thickness of the final double-sided coating was controlled to be 2.0 μm, and the total surface density of the double-sided coating was 1.4 g / m 2 .

[0122] Example 12

[0123] Based on 100 parts by weight of the coatable slurry, 0.6 parts by weight of a PVDF resin having a weight average molecular weight of 900,000-1.1 million and 4.2 parts by weight of a PVDF resin having a weight average molecular weight of 400,000-500,000 were added to dimethylacetamide (DMAC) and dissolved at 50°C for about 3 hours to form an adhesive. Under the conditions of 23°C and 20% relative humidity, the slurry for forming a porous coating was coated on both surfaces of a single-sided ceramic-coated polyethylene porous substrate by a gravure roller (total coating thickness 9 μm, ceramic layer thickness 2 μm, puncture strength 470 gf), coagulated by dimethylacetamide / water coagulation solution (DMAC / water in the coagulation bath = 5:5), washed with pure water, and dried at 80°C to produce a diaphragm having a porous coating. Compared with Example 3, the total thickness of the final double-sided coating was controlled to be 1.6 μm, and the total surface density of the double-sided coating was 1.12 g / m 2 The surface morphology of the diaphragm is shown in Figure 3.

[0124] Example 13

[0125] Based on 100 parts by weight of the coatable slurry, 1.2 parts by weight of a PVDF resin having a weight average molecular weight of 900,000-1.1 million and 3.6 parts by weight of a PVDF resin having a weight average molecular weight of 400,000-500,000 were added to dimethylacetamide (DMAC) and dissolved at 50° C. for about 3 hours to form an adhesive. Under the conditions of 23° C. and 20% relative humidity, the slurry for forming a porous coating was coated on both surfaces of a single-sided ceramic-coated polyethylene porous substrate by a gravure roller (total coating thickness 9 μm, ceramic layer thickness 2 μm, puncture strength 470 gf), coagulated by dimethylacetamide / water coagulation solution (DMAC / water in the coagulation bath = 5:5), washed with pure water, and dried at 80° C. to produce a diaphragm having a porous coating. Compared with Example 3, the total thickness of the double-sided coating obtained was controlled to be 1.2 μm, and the total surface density of the double-sided coating was 0.936 g / m 2 .

[0126] Comparative Example 1:

[0127] Based on 100 parts by weight of the coatable slurry, 3 parts by weight of a PVDF resin having a weight average molecular weight of 900,000 to 1.1 million, 1 part by weight of a PVDF resin having a weight average molecular weight of 200,000 to 500,000, and 7.5 parts by weight of Al2O3 particles were added to dimethylacetamide (DMAC) and dissolved at 50°C for about 3 hours to form an adhesive. Under the conditions of 23°C and 20% relative humidity, the slurry for forming a porous coating was coated on both surfaces of a polyethylene porous substrate (thickness 9 μm, porosity 35%, average pore size 70 nm, puncture strength 450 gf) by a gravure roller, coagulated with a dimethylacetamide / water coagulation solution (DMAC / water in the coagulation bath = 5:5), washed with pure water, and dried at 80°C to produce a separator having a porous coating. The resulting total thickness of the two-sided coating was 2.88 μm, and the total surface density of the two-sided coating was 4.2 g / m 2 .

[0128] Comparative Example 2

[0129] Based on 100 parts by weight of the coatable slurry, 3 parts by weight of a PVDF resin having a weight average molecular weight of 900,000 to 1.1 million, 1 part by weight of a PVDF resin having a weight average molecular weight of 200,000 to 500,000, and 10 parts by weight of Al2O3 particles were added to dimethylacetamide (DMAC) and dissolved at 50°C for about 3 hours to form an adhesive. Under the conditions of 23°C and 20% relative humidity, the slurry for forming a porous coating was coated on both surfaces of a polyethylene porous substrate (thickness 9 μm, porosity 35%, average pore size 70 nm, puncture strength 450 gf) by a gravure roller, coagulated with a dimethylacetamide / water coagulation solution (DMAC / water in the coagulation bath = 5:5), washed with pure water, and dried at 80°C to produce a separator having a porous coating. The resulting total thickness of the two-sided coating was 4.44 μm, and the total surface density of the two-sided coating was 5.64 g / m2 .

[0130] Comparative Example 3

[0131] Based on 100 parts by weight of the coatable slurry, 1.6 parts by weight of a PVDF resin having a weight average molecular weight of 900,000 to 1.1 million, 2.4 parts by weight of a PVDF resin having a weight average molecular weight of 200,000 to 500,000, and 7.5 parts by weight of Al2O3 particles were added to dimethylacetamide (DMAC) and dissolved at 50°C for about 3 hours to form an adhesive. Under the conditions of 23°C and 20% relative humidity, the slurry for forming a porous coating was coated on both surfaces of a polyethylene porous substrate (thickness 9 μm, porosity 35%, average pore size 70 nm, puncture strength 450 gf) by a gravure roller, coagulated with a dimethylacetamide / water coagulation solution (DMAC / water = 5:5 in the coagulation bath), washed with pure water, and dried at 80°C to produce a separator having a porous coating. The resulting total thickness of the two-sided coating was 4.48 μm, and the total surface density of the two-sided coating was 5.06 g / m 2 .

[0132] Comparative Example 4

[0133] Based on 100 parts by weight of the coatable slurry, 1.6 parts by weight of a PVDF resin having a weight average molecular weight of 900,000 to 1.1 million, 2.4 parts by weight of a PVDF resin having a weight average molecular weight of 200,000 to 500,000, and 7.5 parts by weight of Al2O3 particles were added to a dimethylacetamide (DMAC) / water dispersion system with a water content of 6 wt% and dissolved at 50°C for about 3 hours to form a binder. At 23°C and 20% relative humidity, the slurry for forming a porous coating was coated on both surfaces of a polyethylene porous substrate (thickness 9 μm, porosity 35%, average pore size 70 nm, puncture strength 450 gf) by a gravure roll, coagulated with a dimethylacetamide / water coagulation solution (DMAC / water = 5:5 in the coagulation bath), rinsed with pure water, and dried at 80°C to produce a separator having a porous coating. The resulting coating had a total thickness of 4.0 μm on both sides and a total surface density of 4.6 g / m2 on both sides. 2 .

[0134] Comparative Example 5

[0135] Based on 100 parts by weight of the coatable slurry, 2 parts by weight of a PVDF resin having a weight average molecular weight of 900,000 to 1.1 million, 3 parts by weight of a PVDF resin having a weight average molecular weight of 200,000 to 500,000, and 7.5 parts by weight of Al2O3 particles were added to a dimethylacetamide (DMAC) / water dispersion system, wherein the water content was 6 wt%, and dissolved at 50°C for about 3 hours to form an adhesive. Under the conditions of 23°C and 20% relative humidity, the slurry for forming a porous coating was coated on both surfaces of a polyethylene porous substrate (thickness 9 μm, porosity 35%, average pore size 70 nm, puncture strength 450 gf) by a gravure roller, coagulated with a dimethylacetamide / water coagulation solution (DMAC / water in the coagulation bath = 5:5), washed with pure water, and dried at 80°C to produce a diaphragm having a porous coating. Compared with Example 4, the total thickness of the final double-sided coating was controlled to be 4.96 μm, and the total surface density of the double-sided coating was 5.74 g / m 2 .

[0136] Comparative Example 6

[0137] Compared with Example 1, the DMAC / water ratio in the coagulation bath is 8:2. The total thickness of the final double-sided coating is 4.48 μm, and the total surface density of the double-sided coating is 5.32 g / m 2 .

[0138] Comparative Example 7

[0139] Compared with Example 2, the drying temperature was controlled to be 55°C. The total thickness of the double-sided coating obtained was 3.4 μm, and the total surface density of the double-sided coating was 4.62 g / m 2 .

[0140] Table 1 shows the performance indicators of the diaphragms of Examples 1-13 and Comparative Examples 1-7, and Table 2 shows the performance indicators of the corresponding batteries obtained by the diaphragms of Examples 1-10 and Comparative Examples 1-7. It can be seen that despite many influencing factors, when the coating thickness is within a certain range, the bonding strength (N / m) / coating air permeability value (s / 100cc) is ≥ 0.25, and the bonding strength (N / m) / unit coating surface density (g / m 2 / μm)≥10, the coating has more excellent positive electrode bonding performance, high temperature dimensional stability and good air permeability, and thus has a good performance in the electrical performance of the corresponding lithium battery.

[0141] Unless expressly excluded or otherwise limited, each document cited herein, including any cross-referenced or related patent or patent application and any patent application or patent to which this application claims priority or the benefit of, is hereby incorporated by reference in its entirety. The citation of any document is not an admission that it is prior art to any disclosed or claimed herein, or an admission that it, by itself or in combination with any one or more references, presents, suggests, or discloses any such disclosure. In addition, to the extent that any meaning or definition of a term in this disclosure conflicts with any meaning or definition of the same term in a document incorporated by reference, the meaning or definition assigned to that term in this disclosure shall govern.

[0142] Table 1: Relevant performance indicators of the diaphragms of Examples 1-13 and Comparative Examples 1-4

[0143]

Claims

1. A lithium battery separator comprising a porous PVDF-based resin coating, It is characterized in that The porous PVDF resin coating is located on at least one surface of the base film or the ceramic coating film, the thickness of the porous PVDF resin single-sided coating is 0.5-3.5 μm, its bonding strength (N / m) / coating air permeability value (s / 100cc) is ≥0.25, and the bonding strength (N / m) / unit coating surface density (g / m 2 / μm)≥10.

2. The lithium battery separator according to claim 1, It is characterized in that The PVDF resin in the porous PVDF resin coating includes high molecular weight PVDF and low molecular weight PVDF, wherein the high molecular weight PVDF has a weight average molecular weight of 800,000-1.2 million, the low molecular weight PVDF has a weight average molecular weight of 200,000-500,000, and the weight ratio of high molecular weight PVDF to low molecular weight PVDF is 7:1-1:

7.

3. The lithium battery separator according to claim 1, It is characterized in that The porous PVDF-based resin coating may further include inorganic particles. Based on the total weight of the porous PVDF-based resin coating, the content of the inorganic particles is 40-80 wt %.

4. The lithium battery separator according to claim 3, It is characterized in that During the preparation of the porous PVDF-based resin coating, the water content of the slurry is lower than 5wt%.

5. The lithium battery separator according to claim 4, It is characterized in that During the preparation of the porous PVDF-based resin coating, the water content of the coagulation bath is 30wt%-70wt%.

6. The lithium battery separator according to claim 5, It is characterized in that During the preparation process of the porous PVDF resin coating, the drying temperature is 60-100°C.

7. A method for preparing a lithium battery separator comprising a porous PVDF-based resin coating as claimed in any one of claims 1 to 6, It is characterized in that The specific steps are: Step 1: dispersing a PVDF resin and optional inorganic particles in a solvent to prepare a coatable slurry, wherein the solvent includes a good solvent and the water content of the entire slurry is less than 5wt%; Step 2: coating the coatable slurry on at least one surface of the base film, and then immersing the base film in a room temperature coagulation bath to coagulate the coating to form a porous PVDF resin coating, and then washing and drying the coating to obtain a lithium battery separator; The water content in the coagulation bath is 30wt%-70wt%; and the drying temperature is 60-100℃.

8. The preparation method according to claim 7, It is characterized in that The PVDF resin includes high molecular weight PVDF and low molecular weight PVDF, wherein the high molecular weight PVDF has a weight average molecular weight of 800,000-1.2 million, the low molecular weight PVDF has a weight average molecular weight of 200,000-500,000, and the weight ratio of high molecular weight PVDF to low molecular weight PVDF is 7:1-1:

7.

9. The preparation method according to claim 7, It is characterized in that Based on the total weight of the porous PVDF-based resin coating, the content of the inorganic particles is 40-80 wt %.

10. A lithium battery comprising a positive electrode, a negative electrode, a non-aqueous electrolyte and the lithium battery separator according to any one of claims 1 to 6 or the lithium battery separator obtained according to the preparation method of claim 7.

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